The present disclosure concerns a gas turbine engine comprising an exhaust cooling system.
Components of aircraft gas turbines engines such as turbojets and turbofans typically reach high temperatures, and so may require cooling in order to prevent damage to components. In particular, gas turbine engines having afterburners (also known as “reheat”) may require cooling of the exhaust nozzle, to prevent damage to the nozzle itself.
One prior gas turbine exhaust nozzle comprises a cooling channel which surrounds the gas turbine engine core in an annular arrangement (known as a “skirted ring”). Cooling air is provided from a compressor or bypass duct to the channel, which is then exhausted on to a downstream internal cooled surface of the core exhaust nozzle. The cooling air provides a cooling film, which shields the cooled surface from hot exhaust gasses. However, turbulence induced by shear from the relatively slow moving cooling air and the relatively fast (generally supersonic) core exhaust flow causes mixing between the flows, thereby reducing the effectiveness of the cooling. Even where the cooling airflow is provided at high velocity, a relatively slow moving boundary layer is formed adjacent the cooled surface. This slow moving boundary air again interacts with the fast moving core exhaust flow, thereby again causing mixing.
A second prior nozzle comprises a cooling channel contained within the nozzle itself. Small holes are provided which extend into the gas turbine engine core flow. Cooling air thereby leaks out in the main core flow, thereby providing effusion cooling. However, such a system results in a high pressure loss in view of the resistance to flow provided by the relatively small cooling holes. The cooling holes are expensive to manufacture, and are prone to blockages. Furthermore, since the cooling flow enters the main core flow generally normal to the core flow direction, the cooling flow does not contribute to aircraft thrust. Consequently, there is a need for an improved aircraft gas turbine engine exhaust nozzle cooling arrangement which addresses one or more of the above problems.